Powder Fineness (Types of Powder)

The degree of coarseness or fineness of a powder is expressed by reference to the nominal mesh aperture size of the sieves used for measuring the size of the powders. For practical reasons, the use of sieves for measuring powder fineness for most pharmaceutical purposes is convenient but devices other than sieves must be employed for the measurement of particles less than 100 ~ in nominal size.

The fineness of the powder may be expressed as a percentage w/w passing the sieve(s) used or in the following descriptive terms.
The following terms are used in the description of powders.

Coarse powder

A powder all the particles of which pass through a sieve with a nominal mesh aperture of 1700 ~ and not more than 40 percent by weight through a sieve with a nominal mesh aperture of 355 ~.

Moderately coarse powder

A powder all the particles of which pass through a sieve with a nominal mesh aperture of 710 ~ and not more than 40 percent by weight through a sieve with a nominal mesh aperture of 250 ~.

Moderately fine powder

A powder all the particles of which pass through a sieve with a nominal mesh aperture of 355 microns and not more than 40 percent by weight through a sieve with a nominal mesh aperture of 180 microns.

Fine powder

A powder all the particles of which pass through a sieve with a nominal mesh aperture of 180 microns and not more than 40 percent by weight pass through a sieve with a nominal mesh aperture of 125 microns.

Very fine powder

A powder all the particles of which pass through a sieve with a nominal mesh aperture of 125 /lill and not more than 40 percent by weight pass through a sieve with a nominal mesh aperture of 45 microns.

Microfine powder

A powder of which not less than 90 percent by weight of the particles pass through a sieve with a nominal mesh aperture of 45 microns.

Superfine powder

A powder of which not less than 90 percent by the number of the particles are less than 10 micron size.

When the fineness of the powder is described by means of a number, it is intended that all the particles of the powder shall pass through a sieve of which the nominal mesh aperture, in /lill, is equal to that number.

When a batch of a vegetable drug is being ground and shifted, no portion of the drug shall be rejected, but it is permissible, except in the case of assays, to withhold the final tailings, if an approximately equal amount of tailings from a preceding batch of the same drug has been added before grinding.

Sieves

Sieves for testing powder fineness comply with the requirements stated under sieves.

Method

For coarse and moderately coarse powders. Place 25 to 100 g of the powder under examination upon the appropriate sieve having a close-fitting receiving pan and cover. Shake the sieve in a rotary horizontal direction and vertically by tapping on a hard surface for not less 20 minutes or until shifting is practically complete. Weigh accurately the amount remaining on the sieve and in the receiving pan.

For fine and very fine powders. Proceed as described under Coarse and moderately coarse powders except that (a) the test sample should not exceed 25 g, (b) the sieve is to be shaken for not less than 30 minutes or until sifting is practically complete, and (c) the particles passing through a sieve of nominal mesh aperture of 45 micron are determined by suitable microscopic examination.

For microfine and superfine powders. Proceed as described under Fine and very fine powders except that the particles passing through a sieve of nominal mesh aperture of 45 microns (for fine powders.) or those less than 10 microns in size are determined by suitable microscopic examination.

With oily and other powders which tend to clog the openings, carefully brush the screen at intervals during sifting. Break up any lump that may form. A mechanical sieve shaker which reproduces the circular and tapping motion given to sieves in hand sifting but has a uniform mechanical action may be employed.
NOTE: Avoid prolonged shaking that would result in increasing the fineness of the powder during the testing.

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